Chemical Reactions and Equations

Introduction

A chemical reaction is a process in which one or more substances (called reactants) change to form new substances (called products). These changes may involve color change, gas formation, temperature change, or formation of a solid.

1. Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction using chemical formulas.

Example:
Magnesium + Oxygen → Magnesium oxide
Mg + O₂ → MgO

This is a chemical equation showing the reactants and the product.

2. Balanced Chemical Equation
The law of conservation of mass says that mass is neither created nor destroyed in a chemical reaction.
So, the number of atoms of each element must be the same on both sides of the equation.

Balanced example:
2Mg + O₂ → 2MgO

3. Types of Chemical Reactions
Combination Reaction:--
Two substances combine to form one compound.
Example: CaO + H₂O → Ca(OH)₂

Decomposition Reaction:--
A single compound breaks down into simpler substances.
Example: 2H₂O₂ → 2H₂O + O₂

Displacement Reaction:--
One element replaces another in a compound.
Example: Zn + CuSO₄ → ZnSO₄ + Cu

Double Displacement Reaction:
Two compounds exchange their ions.
Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

4. Effects of a Chemical Reaction
Chemical reactions may show:

Change in color
Evolution of gas
Formation of a precipitate
Change in temperature

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Questions on Chemical Reactions and Equations

Showing 91–120 of 167

91.A white solid, when strongly heated, melts and then decomposes to produce a brownish gas and a residue that is yellow when hot and white when cold. Identify the white solid. 92.Why is the reaction between hydrogen gas and chlorine gas to form hydrogen chloride gas considered a combination reaction? 93.During a laboratory experiment, a student mixes two colorless solutions, X and Y. Immediately, a blue precipitate forms. What could be X and Y? 94.What is the significance of the state symbols (s), (l), (g), and (aq) in a balanced chemical equation? 95.Consider the reaction: 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g). If this reaction is carried out in a sealed container, what observation might violate the Law of Conservation of Mass if the container was not sealed? 96.Why are decomposition reactions typically endothermic? 97.When a few drops of phenolphthalein indicator are added to a solution of sodium carbonate, the solution turns pink. What will happen if dilute hydrochloric acid is added drop by drop to this solution? 98.Which of the following is an example of a single displacement reaction? 99.A student performs an experiment where he mixes an aqueous solution of Barium Chloride with an aqueous solution of Sodium Sulphate. i) What is the immediate observation? ii) Write the balanced chemical equation for the reaction. iii) What type of reaction is this? 100.Distinguish between an oxidizing agent and a reducing agent in terms of electron transfer. 101.During the electrolysis of water, if the volume of hydrogen collected is 10 mL, what will be the volume of oxygen collected at the same conditions? 102.A chemist tests two unknown white powders. Powder A, when heated, produces a gas that rekindles a glowing splint. Powder B, when heated, produces a gas that turns limewater milky. Identify Powder A and Powder B. 103.When a student burns a piece of magnesium ribbon in a jar of oxygen, a white powder is formed. If this white powder is then dissolved in water and a drop of red litmus paper is added, what color change will be observed and why? 104.State one difference between displacement and double displacement reactions. 105.Which of the following compounds is a strong oxidizing agent often used in redox titrations? 106.How can you demonstrate experimentally that the reaction of hydrogen and oxygen to form water is a combination reaction? 107.Describe the role of oxygen in both corrosion and rancidity. 108.What is the color change observed when ferrous sulphate crystals are heated? 109.Why is it necessary to balance a chemical equation? Explain with reference to a fundamental law. 110.Predict the product(s) of the following reaction and identify the type of reaction: Al(s) + HCl(aq) → 111.Which of the following reagents can be used to distinguish between magnesium oxide and calcium oxide? 112.A student observed that when dilute hydrochloric acid was added to a piece of magnesium ribbon, the temperature of the test tube increased. What type of reaction is this? 113.Why are certain types of chemical reactions, like photosynthesis and thermal decomposition, considered endothermic? 114.An aqueous solution of metal nitrate P reacts with sodium bromide solution to form a yellow precipitate of compound Q which is used in photography. Q on exposure to sunlight undergoes decomposition to form a metal R and a reddish-brown gas. Identify P, Q, and R. 115.What is the role of manganese dioxide (MnO2) in the decomposition of potassium chlorate (KClO3)? 116.Give an example of a reaction that is both a combination reaction and an exothermic reaction. 117.A student wanted to store fresh potato chips for a long time. What method should they use to prevent them from becoming rancid? 118.Which of the following observations indicates that a chemical reaction has taken place? 119.When silver chloride is exposed to sunlight, it decomposes to form silver metal and chlorine gas. i) Write the balanced chemical equation for this reaction. ii) What type of reaction is this? iii) What is the commercial application of this reaction? 120.Consider the reaction: Fe2O3(s) + 2Al(s) → Al2O3(s) + 2Fe(s). Which of the following statements about this reaction is incorrect?